This invention relates to a wire bonding apparatus for a semiconductor device. More particularly, the invention is concerned with an ultrasonic wire bonding apparatus.
The assembling process for a semiconductor device includes the step of connecting a bonding wire at one end to a pad of the semiconductor pellet and at the other end to an inner lead of the lead frame. Conventionally, a thermal compression bonding was used to bond the bonding wire onto the pad and the inner lead of the lead frame. In this method, it was necessary to heat the bonding portion up to a temperature of 200.degree. C. to 350.degree. C. For this reason, gold was used for the bonding wire, and the lead frame was gold plated. This resulted in an increase in the manufacturing cost of the semiconductor device. On the other hand, for the purpose of reducing the manufacturing cost, an aluminum wire is used as the bonding wire and the lead frame is given a thin gold-plating. This method of decreasing the plating thickness, however, results in a decrease in the bonding strength to the lead frame. To decrease the heating temperature for the bonding portion down to 100.degree. C. from 200.degree. C., an attempt to use an ultrasonic bonding concurrently with the thermal compression bonding was made with respect to the above-mentioned bonding operation. This attempt, however, failed to entirely eliminate the gold-plating for the lead frame. Further, to ensure the low manufacturing cost of the semiconductor device, nickel, iron or tin is plated onto the lead frame, instead of gold for carrying out the ultrasonic bonding. However, since the ultrasonic bonding is effective only in the direction in which the ultrasonic waves oscillate, a conventional bonding apparatus having a single ultrasonic horn coupled to a capillary was unsuitable as the bonding apparatus for complicated semiconductor devices. For example, a semiconductor pellet 2 including an integrated circuit 1 shown in FIG. 2 includes a plurality of pads 3 arranged in the direction A and a plurality of pads 4 arranged in direction B. On the other hand, a lead frame 5 having the semiconductor pellet 2 loaded thereon includes a plurality of inner leads 6 arranged in the direction A and extending in the direction B, and a plurality of inner leads 7 arranged in the direction B and extending in the direction A. Each pad is connected by a bonding wire 8 to an inner lead of the lead frame corresponding in position to the pad. The direction in which the bonding wire 8 is arranged, however, is kept to define a specified angle with respect to a prescribed reference direction. Therefore, when an attempt is made to bond the bonding wire 8 onto every bonding portion by using the bonding apparatus having a single ultrasonic horn effective only in one oscillating direction, it is necessary that the lead frame 5 and the ultrasonic horn of the bonding apparatus be rotated relatively through a wide angular range. Accordingly, it was extremely difficult to automate the bonding operation for a semiconductor device. To briefly explain by way of example, the prior art ultrasonic bonding apparatus necessitates the use of rotating means and displacing means. The former means is designed to rotate the members to be bonded, or the lead frame 5 having the semiconductor pellet 2 loaded thereon at a preselected position, so that a bonding portion thereof is positioned in a prescribed direction. The latter means is designed to displace the capillary having the bonding wire 8 in the X and Y directions to guide the same up to the bonding portion positioned in said prescribed direction. However, these rotating and displacing means are not simple in construction and therefore are not suitable for automation.